A magnetic tunnel junction (MTJ) element as a magnetoresistive element has a stack structure that includes a storage layer having a changeable magnetization direction, a reference layer having a pinned magnetization direction, and an insulating layer disposed between the storage layer and the reference layer. This MTJ element is known to have a tunneling magnetoresistive (TMR) effect, and is used as the storage element of a memory cell in a magnetic random access memory (MRAM).
An MRAM stores information (“1” or “0”) depending on changes in the relative angle between the magnetization directions of the magnetic layers in each MTJ element, and is nonvolatile. As the magnetization switching speed is several nanoseconds, high-speed data writing and high-speed data reading can be performed. In view of this, MRAMs are expected to be next-generation high-speed nonvolatile memories. Further, where a technique called spin transfer torque magnetization switching is used to control magnetization with a spin polarization current, the cell size in an MRAM is reduced so that the current density can be increased. With this, the magnetization of each storage layer can be readily reversed, and a high-density MRAM that consumes less power can be formed.
To increase the density of a nonvolatile memory, a higher degree of magnetoresistive element integration is essential. However, the thermal stability of the magnetic material forming a magnetoresistive element becomes lower with decrease in device size. Therefore, the issue is to improve the magnetic anisotropy and the thermal stability of such a magnetic material.
To counter this problem, MRAMs have recently been formed with perpendicular MTJ elements in which the magnetization directions of the magnetic materials are perpendicular to the film surfaces. Each magnetic material forming a perpendicular MTJ element has a perpendicular magnetic anisotropy. To achieve a perpendicular magnetic anisotropy, a material having a crystal magnetic anisotropy or an interface magnetic anisotropy is selected. For example, FePt, CoPt, and FePd are materials each having a high crystal magnetic anisotropy. Other than the above, there has been a report on an MTJ element that uses MgO as the tunnel barrier layer and a material having an interface perpendicular magnetic anisotropy, such as CoFeB.
The storage layer and the reference layer of an MTJ element each contain a magnetic material, and generate a magnetic field outward. Normally, in a perpendicular magnetization MTJ element in which, the storage layer and the reference layer each have a perpendicular magnetic anisotropy, the magnetic field leakage from the reference layer is larger than that in an in-plane magnetization MTJ element in which the magnetizations of the magnetic materials are parallel to the film surfaces. Also, the storage layer having a lower coercive force than that of the reference layer is greatly affected by the magnetic field leakage from the reference layer. Specifically, due to the influence of the magnetic field leakage from the reference layer, a shift occurs in the magnetization switching field of the storage layer, and the thermal stability becomes lower.
To reduce the magnetic field leakage from the reference layer toward the storage layer in a perpendicular magnetization MTJ element, the following measures have been suggested. The saturation magnetization of the reference layer is lowered, and a magnetic layer (a shift adjustment layer) having such a magnetization direction as to cancel the magnetization of the reference layer is employed.